The present invention relates to a driving method for a liquid crystal display device for use in a liquid crystal display apparatus, etc.
In recent years, liquid crystal display devices have been commercialized in various fields including monitors for personal computers, view finders for video camcorders and projectors. Most of these liquid crystal display devices employ twisted nematic (TN) liquid crystals.
However, the TN-type liquid crystal display devices have a problematic slower response speed and narrower viewing angle.
Incidentally, a color liquid crystal display device with no color filters driven according to a so-called field-sequential scheme has been proposed.
According to the field-sequential scheme, such a color filterless liquid crystal display device is used in combination with, e.g., three light sources of red (R), green (G) and blue (B), which are sequentially turned on to provide the color filterless liquid crystal display device with corresponding color images to be color-mixed in time sequence to form a desired color image. Accordingly, a liquid crystal used is required to possess such a response characteristic that its optical response is completed in each field period for a corresponding color in order to ensure the display of the desired color. As a result, the liquid crystal used for the field-sequential scheme is required to exhibit a higher response speed than ever.
In order to solve the above-mentioned problems, Yoshida et al have proposed a driving method using a combination of a mono-stabilized ferroelectric liquid crystal and a plurality of active (matrix) elements arranged in a matrix form (e.g., Japanese Patent No. 2681528). Such a mono-stabilized ferroelectric liquid crystal provides an electrical optical response characteristic such that the liquid crystal responds to the application of a voltage of one polarity to provide a V-T (voltage-transmittance) curve in a half-V character shape (i.e., a shape given by cutting V-character in half), as shown in FIG. 2 (referred to as a xe2x80x9chalf-V character-mode liquid crystalxe2x80x9d). On the other hand, a TN liquid crystal having an electrical optical characteristic with respect to both a positive polarity voltage and a negative polarity voltage and a liquid crystal as proposed in Japanese Laid-Open Patent Application (JP-A) 9-50049 show an optical response (V-T characteristic) such that the liquid crystal substantially equally responds to both the positive polarity voltage and the negative polarity voltage to provide a V-T curve like a V character (referred to as a (xe2x80x9cV character-mode liquid crystalxe2x80x9d).
FIG. 3 is a sequence (time chart) for driving a liquid crystal display device using a half-V character-mode liquid crystal by using active matrix elements, each adapted for a drive circuit unit as shown in FIG. 4.
Referring to FIG. 4, the drive circuit unit comprises a gate line drive circuit 41, a gate line (row electrode) 42, a source line (column electrode) 43, a source line drive circuit 44, a storage (retention) capacitor Ccs and a liquid crystal capacitor Clc.
According to the driving sequence (time chart) shown in FIG. 3, the entire picture area (panel plane) of the liquid crystal display device is scanned (for writing) six times (first to sixth scannings) in one frame period (e.g., {fraction (1/60)} sec). Specifically, in each frame period, the first scanning (+(R) scanning) is effected for writing (supplying) a data voltage for a red (R) picture (image) of positive (+) polarity, and the second scanning (xe2x88x92(R) scanning) is effected for writing a data voltage for R picture of negative (xe2x88x92) polarity. Thereafter, the third scanning (+(G) scanning) is effected for writing a data voltage for a green (G) picture of (+) polarity, and the fourth scanning (xe2x88x92(G) scanning) is effected for writing a data voltage for G picture of (xe2x88x92) polarity. Further, the fifth scanning (+(B) scanning) is effected by writing a data voltage for a blue (B) picture of (+) polarity, and the sixth scanning (xe2x88x92(B) scanning) is effected for writing a data voltage of (xe2x88x92) polarity. On the other hand, a light source unit comprising three light sources of red (R), green (G) and blue (B) is turned on in such a manner that the R light source is turned on over the first and second scanning periods (for +(R) scanning and xe2x88x92(R) scanning), the G light source is turned on over the third and fourth scanning periods (for +(G) and xe2x88x92(G) scannings), and the B light source is turned on over the fifth and sixth scanning periods (for +(B) and xe2x88x92(B) scannings) sequentially in this order. These scannings are repeated in a succession of frame periods.
In the above sequence, the liquid crystal (half-V character-mode liquid crystal) provides an optical response (V-T characteristic) shown in FIG. 2, so that a color picture (image) display comprising R display state (based on the R data voltage), black state, G display state (based on the G data voltage), black state, B display state (based on the B data voltage), and black state in succession in each frame period is sequentially repeated, thus allowing full-color image display without using color filters.
However, the above-mentioned drive sequence (FIG. 3) is accompanied by problematic crosstalk, since the sequence is performed in a field-inversion drive scheme, wherein the polarity of the applied voltage to each pixel is inverted for each (one) picture scanning period ((+)xe2x86x92(xe2x88x92)xe2x86x92(+)xe2x86x92(xe2x88x92)xe2x86x92(+)xe2x86x92(xe2x88x92)).
More specifically, as shown in FIG. 5, pixel electrodes (defining the pixels) are accompanied by several parasitic (coupled) capacitances. Particularly, coupling the pixel electrode with the source line (data electrode) 43 causes an application of a voltage depending on a display picture (image) to the source line 43, so that a field-through phenomenon attributable to a fluctuation of voltage at the source line 43 occurs, thus leading to a potential fluctuation of the pixel electrodes. As a result, a transmittance (transmitted light quantity) of the liquid crystal display device is also changed, thus failing to obtain a desired gradational characteristic, i.e., the crosstalk phenomenon described below with reference to FIGS. 6 and 7.
FIG. 6 is a plan view for illustrating a mechanism of the crosstalk phenomenon, and FIG. 7 is a time chart for the quantitative explanation thereof.
Referring to FIG. 6, in this embodiment, a white display of a rectangular portion at pixel a and a rectangular portion at a region ranging from pixel b to pixel d is performed on a white background portion on the panel plane of the liquid crystal display device. A gate line I is disposed along the pixels a and b, and a gate line II is disposed along pixels c and d. On the other hand, a source line A is disposed along the pixels a and c, and a source line B is disposed along the pixels b and d. The gate lines I and II are successively scanned in this order.
For the scanning operation, referring to FIG. 7, the gate line I disposed along the pixels a and b is selected at times t3 and t3xe2x80x2, and the gate line II disposed along the pixels c and d is selected at times t5 and t5xe2x80x2. On the other hand, a voltage applied to the source line A disposed along the pixels a and c is +Vw ((+) polarity voltage for writing white data) at a time t2, 0 V at a time t4, xe2x88x92Vw ((xe2x88x92) polarity voltage for writing white data having an absolute value identical to +Vw) at a time t2xe2x80x2, and 0 V at a time t4xe2x80x2. Similarly, a voltage applied to the source line B disposed along the pixels b and d is +Vw ((xe2x88x92) polarity voltage for writing white data) at a time t1, 0 V at a time t6, xe2x88x92Vw ((xe2x88x92) polarity voltage for writing white at having an absolute value identical to +Vw) at a time t1xe2x80x2, and 0 V at a time t6xe2x80x2.
Then, a change in pixel potential with time will be described with reference to FIG. 7 (and FIG. 6).
At the pixel a, the voltage +Vw of the source line A is applied when the gate line I is selected at time t3. Thereafter, an associated gate (of an active element such as thin film transistor (TFT)) is turned off, and a corresponding pixel potential is placed in a high impedance state for holding the voltage +Vw. At that time, however, the pixel potential at the pixel a is affected by field-through phenomenon attributable to a potential fluctuation of an associated line (with which the pixel electrode provides a parasitic capacitance), since there are parasitic capacitances coupled with the pixel electrode as shown in FIG. 5. The pixel potential at the pixel a is not changed until time t4 due to the potential +Vw of the source line A until time t4, but is somewhat dropped from time t4 because the source line A potential is changed to 0 V at time t4. Thereafter, the source line A potential is changed to xe2x88x92Vw at time t2xe2x80x2, and the pixel potential at the pixel a is further dropped. Then, when the gate line I is selected at time t3xe2x80x2, the source line A voltage xe2x88x92Vw is applied at the pixel a. Thereafter, the source line A potential is also changed at time t4xe2x80x2 and at time t2 (in a subsequent (+) field), whereby the pixel potential at the pixel a is also affected at times t4xe2x80x2 and t2. Each of the pixel potentials at the pixels b, c and d is similarly affected by a corresponding fluctuation in source line as shown in FIG. 7 (specific explanation therefor is omitted therein).
Next, the optical response at the pixels will be described.
Referring to FIG. 7, the voltage of +Vw is applied at the pixel a at time t3, whereby transmission state (at the pixel a) is changed to a white state. Thereafter, the resultant transmitted light quantity (transmittance) is changed with time under the influence of the fluctuation of the pixel potential as mentioned above. At the pixel a, the voltage of xe2x88x92Vw is applied at time t3xe2x80x2, whereby the transmission state is changed to block state since the half-V character-mode liquid crystal used provides the V-T characteristic as shown in FIG. 2. Similarly, also at the pixel b, the voltages of +Vw and xe2x88x92Vw are applied at times t3 and t3xe2x80x2, respectively. However, the fluctuation in pixel potential at the pixel b after the voltage applications (of +Vw and xe2x88x92Vw) is different from that at the pixel a, thus resulting in different (time-)integrated transmitted light quantities between the pixels a and b. As a result, a luminance difference is confirmed by the viewer. Further, at the pixel c shown in FIG. 6, an original (intended) display state is black, but an actually displayed picture is white (light source color) due to the crosstalk phenomenon attributable to the pixel potential fluctuation.
Such a crosstalk phenomenon is ordinarily suppressed by a line-inversion drive scheme or dot-inversion drive scheme wherein a polarity of a voltage applied to a source line is inverted for one horizontal scanning period.
In the present invention, however, the liquid crystal display device is driven according to the above-mentioned field-sequential drive method using the field-sequential drive scheme, thus requiring another means or method other than the line- or dot-inversion drive scheme for preventing the crosstalk phenomenon.
A principal object of the present invention is to provide a driving method for a liquid crystal display device having solved the above-mentioned problems.
A specific object of the present invention is to provide a driving method for a liquid crystal display device capable of suppressing the occurrence of crosstalk phenomenon, applicable to a field-sequential drive using a field-inversion scheme, regardless of inversion schemes.
According to the present invention, there is provided a driving method for a liquid crystal display device of the active matrix type comprising: a pair of substrates and a liquid crystal disposed between the substrates so as to form a matrix of pixels provided with a plurality of pixel electrodes arranged in rows and columns, a plurality of row electrodes and a plurality of column electrodes, respectively, for applying a voltage to the pixels via the pixel electrodes, and a plurality of active elements, each provided to a pixel and connected to a pixel electrode, a row electrode and a column electrode, respectively, the driving method comprising driving the liquid crystal display device while inverting a polarity of voltage applied to each pixel every picture scanning field, wherein the voltage applied to each pixel is corrected based on a voltage to be applied to a column electrode connected to the pixel for a period from a time of selecting an active element at the pixel in a field period to a time of selecting the active element in a subsequent field period.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.